1. Field of the Invention
The present invention relates to a filter media for removing liquid airborne particulates from an air stream and, more particularly, to a new and improved filter media for removing liquid paint overspray particles from air streams being exhausted from a paint spray booth, the filter media including both an overspray collection filter and an electret type of electrostatic filter.
2. Background of the Invention
Spray booths are used in a variety of industrial applications for applying paint to many different products. A number of different spray atomizing application techniques can be used in such spray booths. One of these techniques is an air atomization technique wherein coating or paint particles are mixed with an air stream being ejected from a spray gun and the air stream is directed to the product being coated. Another technique is an airless atomization technique wherein the coating material is atomized and propelled by hydraulic pressure to the product being coated. Yet another technique is an electrostatic spraying technique. The product to be coated is grounded and the coating material is atomized (either by an air or airless technique) and is electrically charged. As a result, the coating materials are deposited on the product due to the electrical attraction of charged coating particles to the product being coated.
Regardless of the spray technique that is used, a paint spray booth is commonly employed to contain evaporating solvents and to capture airborne atomized paint particles so as to minimize their impact on the environment and to protect painters from being unnecessarily exposed to the solvents and paint particles used in the coating process, particularly those that may be toxic. In fact, the use of spray booths is normally required for most liquid paint spray applications by federal or state regulatory agencies, including in particular, the Environmental Protection Agency. Moreover, spray booths tend to enhance the quality of the finish being applied to a product being coated by providing a clean environment for the application of liquid coatings to these products.
In such spray booths, it is necessary to maintain a consistent, steady and uniform flow of air throughout the booths. Among other things, the consistent air flow prevents the accumulation of partially dried overspray on an object being coated so that the appearance of the object is not marred and tends to assist in providing the product with a quality finish. Moreover, spray booths prevent the accumulation of hazardous concentrations of potentially explosive solvent vapors. In fact, environmental clean air standards require that the emissions from spray booths must not include more than certain levels of particulates.
To remove paint particulates from the air being exhausted from a spray booth, the common practice is to employ a replaceable fibrous filter which will trap the overwhelming majority of these paint particulates. These filters soon become clogged with such particulates so that the air flow through the spray booth tends to be substantially reduced, thus decreasing the air flow past the worker inside the booth and the products being coated. Moreover, the spray booth has to be shut down to replace such clogged filters (once per eight hour shift is not uncommon) and the fibrous filters are not inexpensive.
While it may not be readily apparent, the capture and retainment of droplets of liquid coatings is technically very different and significantly more complex than filtering dry particles from moving air streams in connection with, for example, a HVAC (heating, ventilating and air-conditioning) system. The spectrum of materials commonly referred to as coatings or paints that are used in a spray booth exhibit a broad range of physical characteristics. Some UV-cured coatings have low viscosities approximating water whereas some commercial high solids coatings have viscosities ranging from molasses to peanut butter. Some coatings dry in seconds at ambient temperatures whereas many thermoset or baking enamels will literally never dry at normal plant ambient temperatures.
Unlike dust and most other dry particles that need to be entrapped in a filter, paint overspray is comprised of wet, atomized, paint droplets typically ranging in size to as large as 30 microns in diameter. As more and more of these particles are being entrapped by an overspray filter, the captured wet particles tend to adhere to each other to the point where they may succumb to the force of gravity and begin migrating down through the filter media. These and other related, unique technical aspects of overspray arrestance demonstrate that the filtration of paint overspray from moving air streams is significantly different and more complex than the filtration of dust and dry particles from moving air streams. In fact, many of the companies in the overspray filtration market are not the same companies in the dust collection and general air filtration markets.
In recent years, advancements in microelectronics has significantly improved the performance capabilities of scientific test instrumentation used in evaluating filtration of overspray paints and the like. In particular, the commercial availability of computerized particle counters has made it possible to measure the functionality of overspray arresting filters in ways not even envisioned a decade ago. For several decades, the only real scientific means for evaluating the relative performance of arresting filters for overspray paints was a, recognized but somewhat (by today's standards) crude, qualitative test procedure commonly known as an "arrestance efficiency" test. This test was and still continues to be conducted by independent filter testing laboratories.
With the increased public focus on environmental issues and the resulting regulations, overspray arrestor filter manufacturers developed what they considered to be more efficient filters. However, neither the manufacturers nor the test laboratories had a means to fully quantify the actual performance of these "more efficient" arresting filters. While existing test procedures found very little difference between various competitive arresting filters, users reported significant functional differences between them.
In the early 1990's, the assignee of the present application, Chemco Manufacturing Company, Inc. of Northbrook, Illinois, initiated an effort to find a new and more effective way to evaluate its arresting filters and, more importantly, to produce performance data that would provide a qualitative basis for its product development process. As a result of this initiative, a test protocol was developed for measuring the efficiency of an overspray filter as a function of the number and size of the actual paint droplets that impinges on a test filter. The data produced by this type of test provided new insights in the arrestance process.
One of the unexpected, but consistent, findings of the new test procedure was that while most overspray arresting filters are fairly effective in stopping the larger sized droplets (i.e., those greater than 10 microns in diameter), a significant amount of smaller diameter particles (i.e., those having a diameter in the range of 0.25-2.5 microns) were present in the air stream being exhausted from a paint booth. The presence of these low mass droplets in the air stream exiting an arresting filter could present a problem because the low mass to surface area ratio of these droplets results in the droplets remaining entrained in the exhaust air stream from the spray booth long enough to be carried out in that exhaust stream and thus expelled into the atmosphere.
Upon this realization, attempts were made to minimize or even to eliminate this phenomenon. Consideration was given to utilizing an externally generated electrostatic field to capture the problematic smaller droplets. However, the paint overspray accumulated on the electrical connection between the generating device and the individual overspray collection filters at an unacceptable rate (the filter had to be changed as frequently as every four hours of use). As a result, the use of an external power supply to maintain an electrostatic field to capture the smaller droplets was not deemed to be practical.
Another way that was considered for entrapping these smaller sized droplets involved the use of an electrostatic field generating concept known in the fibers trade as "electret." Basically, an electret media is considered either a media that contains a permanent electrostatic charge imparted onto the media when it is being manufactured or a media compromised of an appropriate combination of dissimilar fibers which generates an electrostatic charge by virtue of the friction generated when air passes in very close proximity to those dissimilar fibers. In the first case, the media must be constructed of a dielectric fiber capable of holding an electrostatic charge for years. Polypropylene has been proven to be a suitable material for use in such permanently charged media. In the second case, the requisite electrostatic charge is generated by the passage of the air being filtered through a non-woven media fabricated from a blend of dielectric fibers of opposite polarities such as modacrylic and polypropylene. However, it was found that the paint overspray accumulated on this electret media to such an extent that it neutralized the desired electrostatic effect of the electret media at an unacceptable rate when the electret media was used alone as an overspray filter.
Accordingly, it is an object of the present invention to provide a new and improved overspray paint filter media for the exhaust of an overspray paint booth.
It is another object of the present invention to provide a new and improved air filtration media for capturing and retaining airborne particulates including in particular finely atomized droplets of liquid coating materials from an air stream being exhausted from an overspray paint booth.
It is yet another object of the present invention to provide a new and improved a filter media for removing airborne particulates from an air stream being exhausted from an overspray paint booth that includes both an overspray collection filter and an electret type of electrostatic filter.